THE PHYLOGENETIC POSITION AND MORPHOLOGICAL REINTERPRETATION OF TWO ENIGMATIC EDRIOASTEROIDS FROM CANADA

An investigation into the morphology of two enigmatic Canadian edrioasteroids, Thresherodiscus ramosa (Foerste, 1914) from the Middle Ordovician (Caradocian) Verulam Formation of Ontario, and Lispidecodus plinthotus (Kesling, 1967) from the lower Mississippian (Tournaisian) Banff Formation of Alberta, allows their reinterpretation within the phylogenetic framework of Edrioasteroidea. Thresherodiscus was previously thought to be an isorophid edrioasteroid bearing a triradial ambulacral system with atypical branched ambulacra. Previously unrecognized were the unusual mixture of isorophinid (narrow ambulacra with multi-tiered cover plates and complex oral cover plates) and non-isorophinid (multitiered anal pyramid and external expression of ambulacral floor plates) features. More unusual are differentiated adradial interambulacral plates bearing respiratory structures superficially similar to covered diplopores found in a few clades of diploporite cystoids. No similar respiratory structures are known in other edrioasteroids. Lispidecodus was previously classified within a monotypic family Lispidecodidae because of its unusual turreted thecal shape, ambulacra fully separated from the peristomal region, and unusual “tong and U-shaped” ambulacral cover plates. Our investigation reveals the apparent separation of the ambulacra from the peristome is taphonomic, and the ambulacra are arranged into a six-plate pattern common in agelacrinitid edrioasteroids.

Phylogenetic analysis of select isorophid edrioasteroids shows Thresherodiscus to be placed low within the set of most parsimonious trees, but with poor support for any specific placement. The instability of this taxon is based primarily on the mixture of isorophinid and non-isorophinid characters and we suggest tentative classification as a basal isorophid based on the lowest congruent placement in the optimal trees until additional taxa within the lineage leading to Thresherodiscus are discovered. Lispidecodus is placed strongly as a derived discocystinind agelacrinid isorophinid. This placement is supported by tessellate interambulacral plates, advanced hydro-gonopore, and thick ambulacral floor plates. The unusual turreted thecal shape does not warrant classification as a separate monotypic family.